docs: def order_by_descending(self, key):
          """
          Returns new Enumerable sorted in descending order by given key
          :param key: key to sort by as lambda expression
          :return: new Enumerable object
          """
          if key is None:
              raise NullArgumentError(u"No key for sorting given")
          kf = [OrderingDirection(key, reverse=True)]
          return SortedEnumerable3(kf, self._data)